Fuel assembly for a nuclear reactor and a nuclear reactor core comprising such fuel assemblies

ABSTRACT

A fuel assembly in which the fuel rods are arranged in a hexagonal or square, regular pattern, but in which single rods are omitted systematically and deflectors substituted therefor, each deflector normally being situated in a duct, the partition of which consists of six or eight fuel rods, respectively. The deflectors are rigidly connected to grids between the fuel rods so that the rods may expand freely in the grids.

United States Patent 1 1 1 3,787,285

Marstrand Jan. 22, 1974 FUEL ASSEMBLY FOR A NUCLEAR, 2,948,517 8/1960 Cosner 176/78 x REACTOR AND A NUCLEAR RE T 3,379,619 4/1968 Andrews et al 176/76 X 3,393,128 7/1968 Obertelli et a1 ggs ig i ligng SUCH FUEL 3,607,639 9/1971 Santen et al. 176/78 [75] Inventor: Jorgen Marstrand, Gentofte, FOREIGN PATENTS OR APPLICATIONS Denmark 1,434,932 3/1966 France 176/78 1,519,657 5/1968 France 1 Asslgneei Atomenerglkommlsslonen, 942,591 11/1963 Great Britain 176/78 Copenhagen, Denmark [22] Filed; F b, 8, 1971 Primary Examiner-Carl D. Quarforth pp No 113 242 Assistant Examiner-E. E. Lehmann [30] Foreign Application Priority Data [57] ABSTRACT Feb. 13, 1970 Denmark 721 A fuel assembly in which the fuel rods are arranged in a hexagonal or square, regular pattern, but in which 52 us. 01. 176/78, 176/54 Single rods are Omitted systematically and deflectors 51 Int. Cl G216 3/32 Substituted therefor, each deflector normally being 53 Field f Search 176/76, 78 79, 54 55 uated in a duct, the partition of which consists of six or eight fuel rods, respectively. The deflectors are rig- [56] References Cited idly connected to grids between the fuel rods so that UNITED STATES PATENTS the rods may expand freely in the grids.

3,663,367 5/1972 Calvin 176/78 8 Claims, 4 Drawing Figures PATENTEUJANZZIBY-l SHEU 3 OF 3 1 Phl- FUEL ASSEMBLY FOR A NUCLEAR REACTOR AND A NUCLEAR REACTOR CORE COMPRISING SUCH FUEL ASSEMBLIES The present invention relates to a fuel assembly for a nuclearreactor with parallel fuel rods, the points of intersection of the axes of which with a plane perpendicular to the fuel rods form a regular pattern of points which constitutes the corner points in a plane configuration of adjacent, regular figures in the reactor lattice in the form of equilateral triangles or squares and having deflectors, the axes of which are parallel to the fuel rods and impart a vortical motion to the coolant flowing along the deflectors.

An evaulation of the economy of nuclear power stations shows that fuel costs are relatively low, for example, for a boiling heavy water reactor, they are of the order of magnitude of 0.1 cent per kwh. With a fixed price of fissile material, e .g., uranium, it will be possible to achieve savings in fuel through a lower production price, lower enrichment costs and a better burn-up, but it will hardly be possible to significantly reduce the fuel share in the kwh price. The basis of the present invention is the recognition that if material savings are to be achieved, this will first and foremost be by a reduction in the plant cost put in relation to the net effect of the nuclear reactor. As the plant costs do not rise proportionally with the net power, the possibility of increasing the net power exists, but this results in other drawbacks as, for instance, the fact that suspension of operation acquires far greater economic consequences and that smaller countries are not going to be able to fit such big nuclear power stations into their existing electricity supply network.

It is an object of the present invention to provide a fuel assembly permitting a higher energy flux density than hitherto knwon and thereby to reduce the plant costs.

It is the aim of all designers of nuclear reactors to achieve maximum power for a given core size, vide e.g., Danish patent specification NO. 109,158. For boiling water reactors, it isthe so-called burn-up or critical heat flux that has so far imposed the limit.

For reactors operating with natural coolant circulation, the limits for increasing the energy flux density are rather narrow.

It is possible to significantly increase the energy flux density by forced circulation, however, very big and expensive pumps will be required. The maintenance costs of such pumps cannot be estimated until they have been built and worked for some time.

A boiling water reactor is known, having a reactor lattice in which the fuel rods are arranged in a quadratic pattern and where, parallel to the fuel rods in the center of each square, there are deflectors in the form of twisted strips, vide, for example, French patent specification No. l,369,408. This feature provides the possibility of increasing the energy flux density, but the construction is difficult to accomplish in practice.

It is a further object of the present invention to provide an increase in the energy flux density and a structure of greater simplicity and stability.

A fuel assembly according to the invention, is characterized in that in a group of figures in the reactor lattice adjacent to each other, be they groups each consisting of six equilateral triangles, the outer corner points of which, between them, form a regular, hexagonal pattern of points, or groups each; consisting of four squares, the outer corner points of which, between them, form a bigger quadratic pattern of points, there are fuel rods in the outer points of each individual group, while in the common inner point of the group there is-only a deflector.

' This geometrical description of a bundle of parallel fuel rods in a fuel assembly implies that the fuel assembly may be regarded as a bunch of ducts, the wall of which, apart from the marginal zone, consists of eight or six fuel rods, respectively, depending upon whether it is a question of fuel rods that are disposed in the points of intersection of a quadratic or a triangular net pattern.

In a preferred embodiment of the fuel assembly according to the invention, the deflectors pass through and are secured to grids which serve to keep the rods in the transverse direction but permit the rods to move in the longitudinal direction. By constructing the grids with annular parts in which the deflectors are secured, it is possible in this invention to mount the deflectors without regard to which position the deflecting means occupy relative to the axis.

In the following, the invention is explained with reference to the accompanying, purely schematical drawings, in which FIG. 1 shows in horizontal plane view a portion of an embodiment of a fuel assembly according to the invention with parallel, circular cylindrical fuel rods disposed in a hexagonal pattern, six of the rods and the'deflector they surround being shown in section,

FIG. 2 shows a part of a nuclear reactor core composed of a plurality of fuel assemblies like the one illustrated in FIG. 1,

FIG. 3 shows a portion of a fuel assembly with parallel, circular cylindrical fuel rods disposed in a quadratic pattern, and

FIG. 4 shows a perspective view of a part of a fuel assembly.

The fuel assembly, a portion of which is shown in FIG. 1 contains circular-cylindrical fuel rods arranged in a hexagonal pattern in such a way that the outer contour of the fuel assembly insection at right angles to the axes of the fuel rods is hexagonal. This shape renders the fuel assembly well-suited for the construction of a nuclear reactor core, as is schematically shown in FIG. 2, where a plurality of hexagonal fuel assemblies according to the invention are grouped to gether to form a part of a nuclear reactor core.

FIG. 1 shows, for the sake of clearness, a section through six fuel rods ll, 2, 3, 4, 5 and 6 arranged in a hexagonal pattern, and through a deflector 7, The group of six fuel rods forms a duct or passage containing the deflector 7. Since these fuel rods can be of a type known per se, their construction need not be described in greater detail here. They may, by way of example, as the designation in the figure indicates, consist of a circular-cylindrical can which is filled with fissile material.

It will be seen that each of the fuel rods, besides forming part of the group marked by hatching, also forms part of a neighboring group The marginal groups comprise less than six fuel rods.

In the embodiment illustrated in FIG. 1 the deflector 7, which is mounted in the center of the duct formed by the six surrounding fuel rods, is a sectional rod of cruciform cross section, and comprising four helical to the rings.

blades-which impart a vortical motion around the axis of the duct orpassagetoa coolant flowing throughthe vduct. By means of this vertical motion, a separation of steam and water takes place, since the steam, duelto the .vortical motion, moves-towards the axis of the duct or passage, while the liquid moves outwards toward the surrounding fuel rods. The systematic provision of permits a greater energy flux density. v n

The fuel rods and the deflectors which at the top are fastened in .a holder .or the like, not shown, pass space for the steam and in the above described manner through supporting means or grids. 8 mounted at suitable points in the longitudinal direction of the fuel assembly. Spring members .9 and/or other fixing means I are provided on'those parts of the grids surrounding the r'odsin a manner known per se. Theyfix the position 6fthe parallel fuel was in relationto ea h. other but permit displacement of any 'rod in the axial direction. As such'means are.known,'they"are not going'to be denes tjthey are'orily sho inrio, 4 of thedra'wi ngs, in which FI G'.'1I-3 are meant first-'andforemost to show the reactorjlattice, viz. the configuration of the'fuel rods and deflectors in patterns of isoscelesand squares, respectively. These isosceles triangles are preferably, as

shown inFlGsl; in the'form of equilateral triangles.

The deflectors 7 are rigidly connected to the supporting means or' grids, for instance, .by .welding'l0 as shown in FIG.- I, and 4..Consequently, the grids follow the deflectors in their expansion and contraction in the axial direction. The'structure comprising a number of The reason-that employingthe v I ing to theinvention results in a greater energy flux de'ngrids distributed in the axial direction of the assembly structure is easy to construct is mechanically stable and the fuel rods can expand freely in-the axial direction bu't'ja're held: in correct interrelationship by the grids;

1 Those parts .of'th'e'v grids 8, through which the deflectors pass are, in the embodiment shown-,"constructed as circular-cylindrical frings, which facilitates an: easier mounting because the deflectorscan assume, any angutar pps n in thwin nd!!! max/abut ag ns any points of the inside of the rings before theyare welded The height of the gridsin the axialdirection of the fuel assembly is low as compared to the length'of the fuel rods, as they correspondessentially tothe height of conventional grids.

in the example shown, te deflector 7 has four blas, but it is possible toemploy a different number, for example, three or more. The pitch of the helical surfaces can be constant, but it mayalso vary along the axis of and a number of deflectors welde'd to the grids this the deflector in order-to" thereby make'allowance for the varying steam content which increases inthe direction of the topmost end of the fuel assembly.

In FIG. 2, a plurality of fuel assemblies is shown, like the'ones shown in FlGyl, grouped together to form a part of a nuclear reactor core. Four-of the fuel assem blies'are denoted with'referenc'e designations 20, 21,

22,'and 23 At "suitable points" between thefuel assemblies' and preferably in a regular pattern,- control rods arefitted, of which one marked with 24 is'situated between'the fuel units 20, 22 and ,23. The axially movable control rodshave a Yvshapedfcross section,'but are otherwiseof th'e'c'onventional type'and need therefore not,

be described in greatercletail here.

' ln FlG. 3, a'group of eight fuel rods areschematically I shown mounted in a quadratic pattern with-a deflector ensures a good heat transmission and, thereby, prevents burn-out fromtaking place at the loads normally occurring. a

located in the duct or passage formed bythe fuel'rods. The construction of a fuel assembly of groups as shown in FIG. 3 is, moreover, analogous to what has been described in connection with FIG. 1, and the compounding of a plurality of quadratic fuel. assemblies into a quadratic nuclear reactor core is analogous to what has been described in connection with-FIG. 2;-

By employing a fuel-assembly according to the mvention in a boiling light water reactor, an energyflux density is obtained which corresponds essentially to the energy'flux density which can be obtained ina pressure water reactor. A-corresponding increase in the energy flux density occurs in a heavy water reactor. In both cases,'thi s results in smallerdimensions of thereactor core for a given energy'. I I

"Particularly when employingthe fuel; assembly according to the invention in5a-boilingheavy'Waterj'reaQ tor, the economic. advantage will behappreciable' be cause .thereis goingto be a significant drop in the heavy water investment;

sity is because in the first place, the special configuration of the fuel rods anddeflectors, better conditions with respect to'space are created for the steam developed in the course. of ,the operation with the result that a'higher slip ratio is achieved, that is to saythe ratio be-' .tween the steam velocity and the water velocity. The

deflectors result in awater-steam separation, so that-a significant part of the separation is carried out inside the reactor core. I Compared with the said the invention, thanks to' the configuration chosen,--is

easy and inexpensive to construct. The complete assembly constitutes a structure which is stronger when compared with conventional rod bundle fuel cells and, in all other aspects, is equivalent to these.

in comparison with the prior art structure in which the rods are mounted in a quadratic patternof points, the structure accordingtothe prcifint invention has a plurality of through ducts or passages of a relatively large cross section area. The advantage of this is that the flow resistance, that is to say the drop in pressure, is reduced or, conversely, it is possible toachieve higher steamvelocities as compared. to the prior art structure which means higher slip ratios and, conse--- truded sections having, for instance, three .or four blades twisted with a suitable pitch, that is to say a pitch which ensures, in the given conditions, that the flow of steam following the axis of the ductis rid of its water content. The separated water is swirled out in the'dire'ction of the rods, on which it forms a film of water and fuel assembly' accord prior art construction,-it is a substantialadvantage that the structure according to Many different embodiments are feasible within the scope of the invention which is not restricted to those illustrated. The mounting of the fuel rods and the deflectors in a holder at the top may be of any suitable kind known per se, for example, by screwing or clamping.

I claim:

l. A fuel assembly for a boiling water nuclear reactor comprising: a

a plurality of parallel fuel rods arranged in groups in a regular repeating geometric figure,

each group having a center,

an elongated deflector surrounded at a distance by at least six fuel rods for deflecting a steam-water mixture flowing in its axial direction and thereby causing a substantial separation of water from steam by that deflector,

the axis of said elongated deflector containingsaid centers and being generally parallel with axes of the surrounding fuel rods,

a plurality of grids axially spaced apart on the assembly interconnecting said groups so as to permit axial expansion of the rods while supporting them against movement perpendicular to their axes, said grids having a pluraltiy of annular parts into which the deflectors fit,

said elongated deflector passing through said plurality of grids.

2. The assembly of claim 1 in which said geometric figure is an isosceles triangle, each deflector being in the vertex of a group having six surrounding fuel rods, each of which also forms part of an adjacent group.

3. The assembly of claim 1 in which said geometric figure is a square, each deflector being in a group having eight surrounding fuel rods, each of which is also a part of an adjacent group.

4. The assembly of claim 1 in which comprise a plurality of annular parts into which the said annular parts are joined together by segments which define positions for holding said fuel rods.

5. The assembly of claim 1 in which the deflectors are generally helical in shape.

6. The assembly of claim 5 in which the deflectors are helical so as to force water to their peripheries while permitting generally axial passage of steam.

7. The assembly of claim 1 in which the deflectors are of approximately the same length as the rods.

8. The assembly of claim 1 in which the deflectors are extruded sections having at least three vanes twisted in a helical pitch. 

1. A fuel assembly for a boiling water nuclear reactor comprising: a plurality of parallel fuel rods arranged in groups in a regular repeating geometric figure, each group having a center, an elongated deflector surrounded at a distance by at least six fuel rods for deflecting a steam-water mixture flowing in its axial direction and thereby causing a substantial separation of water from steam by that deflector, the axis of said elongated deflector containing said centers and being generally parallel with axes of the surrounding fuel rods, a plurality of grids axially spaced apart on the assembly interconnecting said groups so as to permit axial expansion of the rods while supporting them against movement perpendicular to their axes, said grids having a pluraltiy of annular parts into which the deflectors fit, said elongated deflector passing through said plurality of grids.
 2. The assembly of claiM 1 in which said geometric figure is an isosceles triangle, each deflector being in the vertex of a group having six surrounding fuel rods, each of which also forms part of an adjacent group.
 3. The assembly of claim 1 in which said geometric figure is a square, each deflector being in a group having eight surrounding fuel rods, each of which is also a part of an adjacent group.
 4. The assembly of claim 1 in which comprise a plurality of annular parts into which the said annular parts are joined together by segments which define positions for holding said fuel rods.
 5. The assembly of claim 1 in which the deflectors are generally helical in shape.
 6. The assembly of claim 5 in which the deflectors are helical so as to force water to their peripheries while permitting generally axial passage of steam.
 7. The assembly of claim 1 in which the deflectors are of approximately the same length as the rods.
 8. The assembly of claim 1 in which the deflectors are extruded sections having at least three vanes twisted in a helical pitch. 